75W85 or 75W90?

B.A.Q

I don't really see other than the Red line and Royal Purple for my tranny.

Red Line recommends MT-75W85.
And I only found 75W90 for Royal purple
I drag race almost every week if in the winter, and I plan on taking it to the track when the weather cools off a little bit.

Conclusion of the reviews:
Red Line less performance than RP, better than OEM and most users say after they tried it, it shifts like butter but not the best for long term use or DD I might say.

How would RP harm the tranny in the long term?

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SOLD!! 09 370z Touring MT: GReddy Ti-C CBE - Berk HFC - Stillen 3G CAI
'11 Mustang GT/CS 5.0 MT